<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title></title>
	<script>
		var num=0;
		for(var b=1;b<=3;b++){
			for(var x=1;x<=4;x++){
				var cj=Number(prompt('请输入'+b+'班第'+x+'同学的成绩'));
				num+=cj;
			}
		}		
       	console.log(num/12);
	</script>
</head>
<body>
	
</body>
</html>